In a typical cellular radio communications system (wireless communication system), an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennae in the cells are in turn coupled to a base station controller (BSC), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) or packet data serving node (PDSN) for instance. The switch or gateway may then be coupled with a transport network, such as the PSTN or a packet-switched network (e.g., the Internet).
When an access terminal (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) is positioned in a cell, the access terminal communicates via an RF air interface with the BTS antenna of the cell. Consequently, a communication path is established between the access terminal and the transport network, via the air interface, the BTS, the BSC and the switch or gateway.
With the explosive growth in demand for wireless communications, the level of call traffic in most cell sites has increased drastically over recent years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae. These sectors (which can be visualized ideally as pie pieces) can be referred to as “physical sectors,” since they are physical areas of a cell site. Therefore, at any given instance, an access terminal in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the transport network via the BTS serving that physical sector.
As an access terminal moves between coverage areas, such as sectors, of a wireless communication system, or when network conditions change or for other reasons, the access terminal may “hand off” from operating in one coverage area to operating in another coverage area. In a usual case, this handoff process is triggered by the access terminal monitoring the signal strength of signals that it is receiving in various available coverage areas, and the access terminal or the BSC determining when one or more threshold criteria are met. For instance, the access terminal may monitor signal strength in various available coverage areas and notify the BSC when a given coverage area has a signal strength that is sufficiently higher than the coverage area in which the access terminal is currently operating. The BSC may then direct the access terminal to hand off to that other coverage area.
In some cases, more than one air interface communication protocol might be implemented in a given market area. For instance, as will be described more below, a given market area might provide both legacy CDMA coverage under a standard protocol such as EIA/TIA/IS-2000 Rel. 0, A or other version thereof (hereafter “IS-2000”) and also high data rate coverage according to a standard protocol such EIA/TIA/IS-856 Rel. 0, A, or other version thereof (hereafter “IS-856”). In such a system, an access terminal might not only hand off between coverage areas under a common air interface protocol (e.g., between IS-2000 sectors) but may also hand off between the different air interface protocols, such as between IS-2000 and IS-856. Handoff between different air interface protocols (or, more generally, between different access technologies) is known as “vertical” handoff.
To facilitate vertical handoff, an access terminal operating under a given air interface protocol might be arranged to periodically check for availability of service under another air interface protocol. By way of example, an access terminal operating under IS-856 might periodically check for availability of IS-2000 service. Further, when the signal strength of the available IS-2000 service is sufficiently higher than the signal strength of the existing IS-856 service, the access terminal may hand off from the IS-856 system to the IS-2000 system, typically with approval from the BSC.